1. Field of the Invention
The invention relates to a driving force transmission control apparatus that transmits a driving force in, for example, a vehicle.
2. Description of Related Art
There has been provided a driving force transmission system in a four wheel drive vehicle, in which a dog clutch is arranged between a propeller shaft and a drive source (engine), and a driving force transmission device capable of continuously adjusting a transmitted driving force is arranged between the propeller shaft and auxiliary drive wheels (rear wheels). Refer to, for example, Japanese Patent Application Publication No. 2012-61923 (JP 2012-61923 A).
In the four wheel drive vehicle described in JP 2012-61923 A, if it is determined that the frictional coefficient of a road surface is lower than a predetermined value, the dog clutch is engaged before the four wheel drive vehicle is started, in view of the problem that if slipping occurs at main drive wheels (front wheels) at the time of starting the four wheel drive vehicle, differential rotation between an input member and an output member for the dog clutch increases, and accordingly the dog clutch cannot be engaged. Thus, even if slipping occurs at the main drive wheels at the time of starting the four wheel drive vehicle, the driving force of a drive source can be promptly distributed to the auxiliary drive wheels so as to suppress the slipping.
Further, the driving force transmission device described in JP 2012-61923 A is arranged between an outer housing on the propeller shaft-side and an inner shaft on the auxiliary drive wheels-side. This driving force transmission device includes a multiple disc clutch and a pressing mechanism. The multiple disc clutch includes a plurality of outer clutch plates and a plurality of inner clutch plates, the outer clutch plates and the inner clutch plates being alternately arranged. The pressing mechanism presses the multiple disc clutch so that the outer clutch plates and the inner clutch plates frictionally slide. The pressing mechanism includes a cam mechanism and an electromagnetic coil. In the cam mechanism, a pressing force is generated by relative rotation between a first cam member and a second cam member. The pressing mechanism is configured such that when the outer housing and the inner shaft are rotated relative to each other while the electromagnetic coil is energized, the first cam member and the second cam member are rotated relative to each other to operate the cam mechanism, thereby pressing the multiple disc clutch.
However, in this driving force transmission device, the cam mechanism is operated due to relative rotation between the outer housing and the inner shall, and accordingly, the multiple disc clutch cannot be pressed before the four wheel drive vehicle is started. Thus, even if the electromagnetic coil is in the energized state and the dog clutch is in the engaged state, the four wheel drive vehicle is brought to a two wheel drive mode in which the driving force is transmitted only to the main drive wheels from the driving force source, at the moment at which the four wheel drive vehicle is started.
Thus, it is conceivable to use a pressing mechanism having a configuration in which a cam mechanism is operated by a torque of an electric motor, as in a driving force transmission device described in Japanese Patent application Publication No. 2009-108972. With the use of this pressing mechanism, a multiple disc clutch can be pressed even before a vehicle is started, and accordingly, the vehicle is brought to a four wheel drive mode at the moment at which the vehicle is started. Thus, it is possible to suppress slippage more reliably.
However, the inventors of the present application confirmed that the driving force actually transmitted through the multiple disc clutch varies even when an equal pressing force is generated by the pressing mechanism, in the case of using a pressing mechanism capable of pressing the multiple disc clutch even when there is not differential rotation between the input and output members for the multiple disc clutch. Thus, the inventors earnestly studied the reasons why the above-described phenomenon is caused, and found that there is a correlation between a differential rotation speed between the input and output members for the multiple disc clutch, and the driving force actually transmitted, when the multiple disc clutch is pressed.